Wideband Code Division Multiple Access (WCDMA) telecommunication systems have many attractive properties that can be used for future development of telecommunication services. In particular, the enhanced uplink of the WCDMA system is one ingredient in the mobile broadband solution of WCDMA. Now, in order to retain stability of a WCDMA cell, possibly a cell running enhanced uplink, the load needs to be kept below a certain level. This follows since the majority of uplink user channels, at least in WCDMA, are subject to power control. This power control aims at keeping the received power level of each channel at a certain signal to interference ratio (SIR), in order to be able to meet specific service requirements.
Since the Radio Base Station (RBS) tries to keep each channel at its specific preferred SIR value, it may happen that an additional user, or bursty data traffic of an existing user, raises the interference level, thereby momentarily reducing the SIR for the other users. The response of the RBS is to command a power increase to all other users, something that increases the interference even more. Normally this process remains stable below a certain load level. In case a high capacity channel would suddenly appear, the raise in the interference becomes large and the risk for instability, a so called power rush, increases. It is thus a necessity to schedule high capacity uplink channels, like the enhanced uplink (EUL) channel in WCDMA, so that one can insure that instability is avoided. In order to do so, the momentary load must be estimated in the RBS or any node connected thereto. This enables the assessment of the capacity margin that is left to the instability point.
One approach to improve load estimation is disclosed in the published international patent application WO 2006/076969. A minimum value of a power quantity, preferably a difference between the instantaneous total received wideband power and the instantaneous sum of powers of all links used in the same cell, is used as an estimate of an upper limit of the thea mal noise floor, based on which a noise rise measure can be estimated. An optimal and soft algorithm for noise rise estimation based on a similar basic idea of minimum values is disclosed in the published international patent application WO 2007/024166.
The instantaneous load was in such an approach obtained from two load measures addressing coverage and cell stability. The load measure for coverage was obtained by the use of a dedicated algorithm which estimates the rise over thermal (RoT), from measurements of the received total wideband power (RTWP). The cell stability was obtained by a so called load factor calculation, exploiting channel powers subject to fast inner loop power control.
To handle increasing uplink data rates, interference cancellation (IC) is being introduced in WCDMA. A conventional procedure to perform IC is summarized by the following steps. A channel model of the interferer to be cancelled is estimated. This does not cause any additional operations, since this channel model is anyway needed. The transmitted signal of the interferer to be cancelled is also decoded. This is also anyway needed. A replica of the received signal of the interferer to be cancelled is then created, by use of the channel model and the decoded signal. This replica may e.g. be reconstructed as an IQ chip stream. The replica of the interfering signal is subsequently subtracted from the received signal of the user to be decoded, thereby hopefully reducing the remaining power of the interferer to very low power levels.
This procedure obviously affects the load measurement functionality of the WCDMA EUL. Thus, the scheduler must be aware of the instantaneous uplink load in several signal points of the new IC receiver structure in order to be able to utilize the entire amount of resources. Unless such load estimates are made available it will not be possible to exploit the link gains fully, when scheduling EUL users. In the published international patent application WO 2008/097145, load estimation with IC of this conventional type is handled in analogy with earlier noise rise estimation procedures.
Another approach to limit the effect of interference is to use some kind of interference whitening approaches, such as GRAKE, GRAKE+ or chip equalizer. The RAKE receiver is the standard receiver in WCDMA systems, see e.g. H. Holma and A. Toskala, WCDMA for UMTS—Radio Access for Third Generation Mobile Communications. Chichester, UK: Wiley, 2001, pp. 27-30. The GRAKE+ receiver is an extension of the RAKE receiver in that an additional step is introduced that transforms the received signal to one that is spectrally white, before RAKE processing is performed, see e.g. L. Fulghurn, D. A. Cairns, C. Cozzo, Y.-P. E. Wang and G. E. Bottomley, “Adaptive generalized Rake reception in DS-CDMA systems, Submitted to IEEE Trans. Wireless Commun., 2008. The basic difference with GRAKE+ as compared to conventional RAKE, is that each user sees a reduced level of interference, immediately after the weight combining step. In GRAKE+, a covariance matrix {circumflex over (R)}u, u=1, . . . , U, with an order equal to the number of fingers is first estimated to capture the interference. The codes not used by the present user u may be used in order to estimate the covariance matrix {circumflex over (R)}u. The GRAKE+ receiver thus uses an estimated covariance matrix {circumflex over (R)}u that models the interference for computation of the combining weights for the users u, u=1, . . . , U. Expressed mathematically, it can be written:{circumflex over (R)}uŵu=ĥu, u=1, . . . ,U,  (1)where ĥu, u=1, . . . , U, is the net channel response of user u and where ŵu are the combining weights. The effect of (1) is that GRAKE+ essentially whitens the correlated interference and removes large spectral peaks from interferers at certain finger locations and for certain antenna elements. Thus, in GRAKE+, the user experiences a reduced level of interference, immediately after the weight combining step. Note that GRAKE+ is still a linear receiver.
In order to utilize the reduced interference provided by the GRAKE+ receiver, the load estimations have to be performed taking the interference whitening of the GRAKE+ into account. The availability of load estimates is important for the admission and congestion control functionality of the RNC. The reason being that these functions will otherwise limit the exploitation of the link gains associated with the interference cancelling or interference cancelling receivers. However, in contrary to what is valid for interference cancellation based on regeneration and subtraction, the thermal noise floor is changed in the same manner as the interference reduction obtained by the GRAKE+ process, and can no longer be treated as constant after interference whitening. In order to measure the instantaneous load, accounting for IC gains, it is necessary to address the load after the GRAKE+ interference whitening step. A treatment analogue of WO 2008/097145 can therefore not be used for achieving a noise rise estimation, the reason being that WO 2008/097145 requires the noise floor to be constant. There is thus a problem of using the created reduced interference, since no reliable noise rise estimation is available. Similar problems are present for systems utilizing GRAKE, where sub-blocks of the covariance matrix of (1) are used.
Further more, as briefly mentioned above, a rise over thermal (RoT) measure, typically available from measurements of the received total wideband power (RTWP) when the GRAKE+ influence is taken into account, is useful for addressing cell coverage. However, for the purpose of obtaining a highly reliable cell stability, such RoT measures are not as effective since powers that are not subject to fast inner loop power control are included in the analysis.
There is a related type of interference whitening receiver for WCDMA which is also linear, denoted the chip equalizer. The difference between GRAKE+ and the chip equalizer is simply the order of certain basic operations. The consequence is that the problems of providing a reliable noise rise measure for cell stability are valid for the systems using the chip equalizer as well.